1. Field of the Invention
The present invention is related to a switching power supply, and more particularly, to a switching power supply which updates the limit signal of the present period according to the sampled value of the reference signal during the previous period.
2. Description of the Prior Art
A well-designed power converting circuit aims at providing a stable output voltage and a wide range of output current. Therefore, when a sudden variation in the load occurs, efficient conversion can still be achieved by keeping the output voltage at its original voltage level and swiftly providing various load currents accordingly. Common power converting circuits include switching regulators and linear regulators.
Reference is made to FIG. 1 for a diagram illustrating a prior art switching power supply 100. Switching power supply 100, including a transformer 102, a power switch 104, an adjusting circuit 106, a detecting circuit 108, and a limit signal generator 110, can converts input voltage VIN into output voltage VOUT. Power switch 104 operates according to a switch control signal VSW: when power switch 104 is turned on, the current passing the primary coil of transformer 102 increases since transformer 102 is charged by input voltage VIN; when power switch 104 is turned off, the energy stored in the primary coil of transformer 102 is released by charging output capacitor Co through the secondary coil of transformer 102. Detecting circuit 108, including a resistor RCS coupled in series with power switch 104, can monitor the current passing power switch 104, thereby providing a corresponding detecting signal VCS. Limit signal generator 110 can provide a limit signal VLIMIT for approximately defining the maximum current IP-MAX which is allowed to pass the primary coil of transformer 102. Adjusting circuit 106 includes a comparator 114 and a driving circuit 116. When detecting signal VCS exceeds limit signal VLIMIT, driving circuit 116 generates switch control signal VSW according to a compare signal VCOMP so as to turn off power switch 104. At the moment when power switch 104 is turned off, the current passing power switch 104 is equal to IP-MAX.
Since the maximum output power of switching power supply 100 is determined by the maximum current IP-MAX, the value of IP-MAX should remain constant regardless of input voltage VIN. However, signal propagation delay inevitably exists from that detecting signal VCS exceeds limit signal VLIMIT to that power switch 104 is actually turned off. Therefore, if limit signal VLIMIT is fixed, the maximum current IP-MAX will increase with input voltage VIN. One common solution is to make limit signal VLIMIT increase with the turn-on time of power switch 104. In other words, a higher input voltage VIN, resulting in a shorter turn-on time of power switch 104, corresponds to a smaller value of limit signal VLIMIT. With proper design, signal propagation delay can be compensated and a stable maximum current IP-MAX can thus be provided.
Another common problem found in switching power supplies is sub-harmonic oscillation. Sub-harmonic oscillation occurs when the duty cycle is over 50% and the switching power supply may oscillate with half the frequency with which the power switch is turned on/off. Slope compensation is a solution to solving sub-harmonic oscillation. One simple approach is to make limit signal VLIMIT decrease with the turn-on time of power switch 104. In other words, a larger duty cycle, resulting in a longer turn-on time of power switch 104, corresponds to a smaller value of limit signal VLIMIT.
As mentioned above, the approach aimed at improving sub-harmonic oscillation worsens the variation in the maximum current IP-MAX caused by signal propagation delay, and vice versa.